This invention relates to whistles as used for signalling purposes. Such whistles are used for a variety of purposes, but it will be convenient to hereinafter describe the invention with particular reference to whistles as used for distress signalling purposes.
Whistles of the foregoing kind are commonly attached to life jackets to provide a means of producing an audible signal if the wearer of the jacket is in a distress situation. Such whistles are also carried by hikers, campers and others who might be caught in difficult circumstances in a remote location.
A problem with whistles of the foregoing kind is that they do not project the generated sound over a sufficient distance and/or do not generate sound of a level such as to be audible over a range of circumstances.
It is an object of the present invention to provide an improved whistle of the foregoing kind. It is a particular object of the invention to provide such a whistle which is able to transmit a signal over a relatively large distance and at a noise level such that the potential for detection is relatively high.
A whistle according to the invention is characterised in that it includes at least two sound producing chambers which are arranged to produce relatively high and low frequency sounds respectively. In a preferred arrangement, the sound range of the whistle is such as to substantially cover the upper and lower limits of human hearing. For example, the frequency range of the whistle may be between 2 and 8 kilohertz with a maximum decibel rating of 113 decibels at 2.5 kilohertz.
An advantage of high frequency sound is that it tends to cut through background interference, but it has the disadvantage of dispersing rapidly and therefore travels over a limited distance. Low frequency sound on the other hand travels over a relatively long distance, but tends to be masked by background noise. A whistle of the foregoing kind therefore has the advantage of producing sound which travels well and is also discernible over background noise.
The foregoing characteristics can be embodied in whistles of various forms. It will be convenient, however, to hereinafter describe the invention with reference to one particular form of whistle.